A phase locked loop is a well known circuit which typically includes a phase detector, a filter, and a voltage controlled oscillator (VCO). A stable input signal is applied to the phase detector which compares the input signal to the output of the voltage controlled oscillator. The output signal of the phase detector is representative of the phase difference between the input signal and the output of the voltage controlled oscillator. The output signal of the phase detector is filtered. The filtered signal is used as an error signal to control the VCO, thereby causing the frequency of the VCO to track the frequency of the stable input signal.
It is well known to those skilled in the art to generate different signals having a carefully determined frequency (f.sub.vco) using a frequency synthesizer having a PLL 10 as illustrated in FIG. 1. The PLL 10 includes a controllable oscillator (VCO) 14 locked to a crystal source which provides an input signal f.sub.xtal. The frequency of the VCO signal f.sub.vco is typically divided by a controllable divider 15 having a division number N to obtain a signal having a frequency f.sub.v. The signal f.sub.v is thereafter compared to the reference signal f.sub.ref which is derived from the division of the input signal f.sub.xtal from the crystal frequency source by a frequency divider 11 having a division number R. The comparison of the signals having frequencies f.sub.ref and f.sub.v in the phase detector 12 generates the control or error signal E. The control signal E is filtered by the filter 13 in order to remove signal components emanating from the signals F.sub.ref and f.sub.v respectively. The filtered signal U controls the VCO 14 so that a balance condition is reached (f.sub.vco =f.sub.xtal .times.N/R). By choosing different division numbers, N and R respectively, different frequencies can be achieved with a relatively high degree of accuracy.
In this type of frequency synthesizer, the frequency can be changed by selecting the division numbers R and N. After a new frequency has been selected, a certain time is required to achieve balance. This time is determined by the filter 13 among other things. In many implementations it is necessary to achieve quickly a stable output signal f.sub.vco. Accordingly, the filter 13 has to be designed to have a relatively broad bandwidth.
The bandwidth of the filter 13 in relation to the reference frequency f.sub.ref also determine how large a disturbance from f.sub.ref and f.sub.v will leak through the VCO 14. Consequently, the filter 13 and the reference frequency f.sub.ref also determine the level of disturbances in the VCO output signal f.sub.vco. The smallest channel spacing has to be equal to or larger than the reference frequency f.sub.ref. The requirements for a pure signal, therefore, are in conflict with the requirements for a relatively quick lock-in and relatively tight channel spacing.
In addition to the disturbances emanating from the phase detector 12, there are additional disturbances in the signal f.sub.vco. These disturbances can constitute noise from the VCO 14. For certain frequency bands it can be difficult to design a resonator of the VCO sufficiently inexpensive and small enough to achieve low disturbances and to meet desired performance specifications.
When the synthesizer output signal covers a comparatively broad frequency range, the division of the signal in two or more separate signals having a fixed phase difference (e.g., 90.degree.) becomes difficult. Such phase shifted signals are often required for a vector I/Q modulation.
In the past, attempts have been made to increase the frequency range of frequency synthesizers by adding a frequency multiplier to the output of the voltage controlled oscillator. Swedish Patent No. 441,719 discloses another technique in which the frequency range is increased by using a frequency divider coupled to the output of the VCO. Neither of these two proposed solutions have been able to overcome completely the above-described problems associated with frequency synthesizers.
Accordingly, there is a need for a new improved frequency synthesizer of the phase locked loop type which can achieve quick lock-in combined with low noise/interference levels. The improved frequency synthesizer must be capable of switching reference frequencies without generating phase jumps.